My imported blender building in unity has loads with missing walls!
The problem is probably that your normals are flipped the wrong way, Unity doesn't render normals that aren't facing a camera. In your blender file. In edit mode go to mesh Normals and it can flip and recalculate your object normals there. The face or faces have to be selected for the tool to work.
I hope that helps, others have had this problem too.